I was suffering from coughing problem from 27th of july, and the cough was dry and there wasn't any kind of phelgam coming out. Suddenly I noticed from the last 3 days that while I cough , a phelgam comes out and the colour of the phelgam is white with dark black. And there is no sign of shortness of breath and chest pain. I am just  curious to know the reason of the change of the colour of phelgum which dark black with white mixed. Is there any serious issue regarding this matter. I also consulted with a doctor who asked me to do X-ray of chest and Sputum for AFB. I want good suggestions and advices from the doctors. Kindly help me regarding this matter